Ignorance mostly.
Honestly, I've only met between 6-10 individuals whose parents where from the Horn of Africa but I've never seen them to date black people of West African descent.
Feel free to educate me tho, cause I do have a super small sample.
Most are just unfamiliar with other groups when they get here. Just learning things like "Hello" & "How are you" in their language will get them talking. America is seen as the "prize" to get to so YOU saying something in THEIR language says you care and think they are important people.